During the 1980's a marked advance took place in silver halide photography based on the discovery that a wide range of photographic advantages, such as improved speed-granularity relationships, increased covering power (both on an absolute basis and as a function of binder hardening), more rapid developability, increased thermal stability, increased separation of native and spectral sensitization imparted imaging speeds, and improved image sharpness in both mono- and multi-emulsion layer formats, can be achieved by employing thin, high aspect ratio tabular grain emulsions. These advantages are demonstrated in Kofron et al U.S. Pat. No. 4,439,520.
An emulsion is generally understood to be a "thin, high aspect ratio tabular grain emulsion" when tabular grains having a thickness (t) of less than 0.2 .mu.m account for at least 50 percent of total grain projected area and the tabular grains have a mean aspect of greater than 8. A grain is easily visually recognized to be a tabular grain when it contains two parallel major faces that are substantially larger than any remaining faces. Quantitatively, a grain with parallel major faces is generally considered to be a tabular grain when its aspect ratio, the ratio of its equivalent circular diameter (ECD) to its thickness (t), is at least 2.
Initially practical interest in tabular grain emulsions centered on applications in camera speed taking films and in indirect radiography (radiographic imaging in which the tabular grain emulsions are image-wise exposed by light emitted from intensifying screens when the screens are exposed to X-radiation). For camera speed films silver iodobromide emulsions have been traditionally preferred and for radiographic films silver bromide emulsions (optionally containing up to about 3 mole percent iodide) have been preferred. The thin, high aspect ratio tabular grain emulsions that have served these applications have contained tabular grains having opposed {111} major faces. The {111} major faces of the tabular grains exhibit a three-fold symmetry, appearing hexagonal or triangular.
Relatively recently interest has increased in the photographic art in combining the known advantages of thin, high aspect ratio tabular grain emulsions with the advantages of high chloride grain structures. The term "high chloride" as applied to grain structures and emulsions is herein employed to indicate at least 90 mole percent chloride, based on silver. The advantages that high chloride emulsions offer over those of other halide compositions include lower native blue sensitivity (thereby contributing to the lower color contamination when used as green or red recording emulsion layers), more rapid development rates, and rapid fixing with ecologically preferred sulfite ion fixers.
It was recognized from the outset that quite different emulsion preparation strategies must be practiced to obtain tabular grain emulsions of differing halide content. Although Kofron et al disclosed high chloride tabular grain emulsions, a difficulty that was encountered is that silver chloride exhibits a strong preference for forming grain structures with {100} crystal faces. The high chloride tabular grain emulsions disclosed by Kofron et al exhibit {111} major faces. The use of high chloride {111} tabular grain emulsions has been hampered by the requirement to employ a morphological stabilizer to prevent high chloride {111} tabular grains from reverting to nontabular forms.
Mignot U.S. Pat. No. 4,386,156 (summarized in column 17 of Kofron et al) discloses the preparation of silver bromide tabular grains with {100} major faces. Saito EPO 0 569 971 discloses modified forms of {100} tabular grains containing at least 25 mole percent bromide.
Relatively recently thin, high aspect ratio, high chloride tabular grain emulsions have been discovered that exhibit {100} crystal faces. By preparing high chloride tabular grains for the first time in an inherently stable crystal form, the complications of morphological stabilizers have been eliminated and remarkable levels of photographic performance have been observed. The sensitivities of these emulsions have approached the sensitivity levels of the more efficient silver iodobromide emulsions. These thin, high aspect ratio, high chloride {100} tabular grain emulsions are illustrated by Maskasky U.S. Pat. Nos. 5,275,930 and 5,292,632; House et al U.S. Pat. No. 5,320,938; Szajewski et al U.S. Pat. Nos. 5,310,635 and 5,356,764; and Brust et al U.S. Pat. No. 5,314,798.
K. Endo and M. Okaji, "An Empirical Rule to Modify the Crystal Habit of Silver Chloride to Form Tabular Grains in an Emulsion", J. Photographic Science, 1988, Vol. 36, (1988), pp. 182-189, set out to produce an empirical rule for selecting materials for use as grain growth modifiers in preparing silver chloride tabular grain emulsions by double-jet precipitation. The rule was tested by adding various ligands, CN.sup.-, SCN.sup.-, I.sup.-, (S.sub.2 O.sub.3).sup.-2, (SO.sub.3).sup.-3 and thiourea (including derivatives) to 3M sodium chloride solutions at concentrations of 0.001, 0.005, 0.01 and 0.1M. The 3M sodium chloride solution was then used with 2M silver nitrate in double-jet precipitations. Tabular grains having {100} and {111} faces were produced. Based on these investigations Endo et al concluded that to be useful as a grain growth modifier in forming tabular grain high chloride emulsions the first formation constant of the ligand, .beta..sub.1 (L), must be more than .beta..sub.2 (Cl.sup.-)--i.e., .beta..sub.2 (Cl.sup.-)/.beta..sub.1 (L) must be less than unity (one). In Table 2 Endo et al reported .beta..sub.2 (Cl.sup.-)/.beta..sub.1 (L) for SCN.sup.- to be 6.3, thereby indicating SCN.sup.- not to be suitable for use as a grain growth modifier. In FIG. 7 Endo et al shows a relatively thick silver chloride grain population produced using 0.10M KSCN.
Maskasky U.S. Pat. No. 5,061,617 discloses employing thiocyanate as a grain growth modifier for the formation of high chloride tabular grains having {111} major faces.
Maskasky U.S. Pat. No. 5,399,477 is directed to a photographic element containing an emulsion with silver halide grains having two parallel {100} major faces and {111} or {110} corner faces or {110} side faces that are formed by non-epitaxial deposits that protrude from the {100} major faces.